Hydraulic fluids with erosion resistance

ABSTRACT

The erosion resistance of phosphate ester based hydraulic fluids has been found to be enhanced by incorporating in the fluid a minor amount of an alkali metal perfluorophosphate and a mono epoxycyclohexane carboxylate.

Continuation-in-Part of U.S. Ser. No. 10/703,987 filed Nov. 7, 2003 which is based on Provisional Application 60/426,946 filed Nov. 15, 2002.

FIELD OF INVENTION

The present invention is concerned with enhancing the anti-erosion properties of phosphate ester based fluids used in transmitting power in hydraulic systems.

BACKGROUND OF INVENTION

Hydraulic fluids are used in a wide variety of industrial applications, including control systems for jet turbines, steam turbines, aircraft and aerospace equipment. For example, in aircraft equipment they are used as the power transmitting medium in operating various moving parts of the aircraft, such as wing flaps, ailerons, rudders and landing gear.

The proper functioning of such important control mechanisms dictate that the hydraulic fluids meet stringent performance requirements including thermal stability, fire resistance, hydrolytic stability and elastomer compatibility, to mention a few.

Organic phosphate ester fluids have been recognized as a preferred base stock for use in hydraulic fluids. Indeed, in present commercial aircraft hydraulic fluids, phosphate esters are among the most commonly used base stocks.

In order to meet the many performance criteria established for hydraulic fluids, and especially aircraft hydraulic fluids, it is common practice to incorporate various additives in: base stocks. For example, experience has shown that orifices in the servo control valves of aircraft hydraulic systems are subject to erosion which is attributed to streaming current induced by fluid flow. Valve orifice erosion, if extensive, can greatly impair the functioning of the valve as a precise control mechanism. Therefore, various additives have been used in hydraulic fluids as erosion inhibitors.

Nonetheless there remains a need for increased choice of useful erosion inhibitors, especially for improved erosion inhibitors for phosphate ester based hydraulic fluids.

SUMMARY OF INVENTION

It has now been discovered that by incorporating a minor amount of both an alkali metal perfluorophosphate and a mono epoxycyclohexane carboxylate into a major amount of a phosphate ester based hydraulic fluid the anti erosion properties of the composition are surprisingly enhanced. Thus, in one embodiment there is provided a composition comprising:

a major amount of a phosphate ester base stock; and

a minor amount of an alkali metal perfluoro phosphate and a mono epoxycyclohexane carboxylate.

DETAILED DESCRIPTION OF INVENTION

The compositions of the present invention comprise a major amount of a phosphate ester base stock. The phosphate esters typically employed in such base stocks include trialkyl phosphates, dialkylaryl phosphates, and triaryl phosphates having alkyl groups of from 3 to 8 and preferably 4 to 5 carbon atoms. Such phosphate ester base stocks are the preferred base stocks of the instant invention. Particularly preferred phosphate esters are trialkyl phosphate esters and among these tri-n-butyl phosphate and tri-isobutyl phosphate are preferred.

The amount of each type of phosphate ester comprising the base stock can vary depending upon the type of phosphate esters involved. The phosphate ester base stock used in the present invention comprises, based on the total weight of the base stock, (a) from about 10 wt % to about 100 wt %, and preferably from about 20 wt % to about 90 wt % of a trialkyl phosphate; (b) from 0 wt % to about 75 wt % and preferably from 0 wt % to about 50 wt % of a dialkylaryl phosphate; (c) from 0 wt % to about 3 wt %, and preferably from 0 wt % to 10 wt % of an alkyl diaryl phosphate; and (d) from 0 wt % to about 20 wt % and preferably from 0 wt % to about 15 wt % of a triaryl phosphate.

The composition of the present invention includes a minor but effective amount of both an alkali metal perfluoro phosphate and a mono epoxy cyclohexane carboxylate.

Specifically the composition will include from about 0.01 wt % to about 0.5 wt % based on the weight of the composition, of an alkali metal perfluoro phosphate or mixtures of alkali metal perfluorophosphate. Alkali metals, of course, include lithium, sodium, potassium, rubidium and cesium. Preferably the alkali metal perfluorophosphate is potassium perfluoro phosphate.

The composition of the present invention also will include a mono epoxycyclohexane carboxylate. Specifically the composition will include from about 1 wt % to about 10 wt % of a mono epoxide represented by the formula

where R is selected from the group consisting of alkyl groups having from 1 to 10 carbon atoms, and —(CH₂)_(n)—OR₁ where R₁ is an alkyl group of 1 to 10 carbon atoms and n is an integer of from 1 to 3. Preferably R is a branched alkyl group of 4 to 8 carbon atoms and more preferably R is a 2-ethyl-1-hexyl group.

In a preferred embodiment the hydraulic fluid compositions of this invention further comprise an antioxidant or mixture of antioxidants in an amount effective to inhibit oxidation of the fluid or its components. Suitable antioxidants include polyphenols, trialkyl phenols and di(alkyl phenyl)amines. Representative examples include: bis(3,5-di-tert-butyl-4-hydroxy phenyl)methane (e.g., Hitec® 4702 from Ethyl Corp.); 1,3,5-trimethyl-2,4,6-tris(3,5-di-tert-butyl-4-hydroxy phenyl)benzene (e.g., Ethanox® 330 from Albemarle Corp.); tetrakis (methylene (3,5-di-tert-butyl-4-hydroxy hydrocinnamate) methane (e.g., Irganox® 1010 from Ciba Geigy); di(n-octylphenyl)amine (e.g., Vanlube 81 from Vanderbilt); and the like.

The antioxidants or mixture of antioxidants typically will be used in amounts ranging from about 0.5 wt % to about 3 wt % based on the total weight of the composition.

In another preferred embodiment the compositions of this invention further comprise an effective amount of a rust inhibitor or a mixture of rust inhibitors. Representative examples of rust inhibitors include calcium dinonylnaphthalene, a Group I or Group II metal over based sulfurized phenate the like. Typically the rust inhibitor or mixture thereof is employed at from about 0.001 wt % to about 1 wt % based on the weight of the composition.

In yet another embodiment the compositions further comprise a viscosity index improver (VII). Especially preferred VII's are alkyl poly acrylates and alkyl polymethacrylate ester polymers in which at least 85% by weight have a molecular weight in the range of about 50,000 to about 100,000, and where the alkyl groups are preferably n-hexyl and isodecyl.

The composition of the invention can optionally contain further additives such as copper corrosion inhibitors, anti foaming agents, dyes and the like.

EXAMPLES Example 1 and Comparative Example 1

A fluid composition was prepared by blending 66.8 wt % of tri-n-butyl phosphate, 12.1 wt % alkylated triphenyl phosphate; 6.2 wt % of a VI improver; 5.7 wt % of a mono epoxycyclohexane carboxylate; 0.05 wt % KPF₆; and the balance antioxidants, rust inhibitors dye and defoamer. The fluid was then tested in an electro-deposition test developed by Boeing Aircraft Company that simulates the tendency of deposit formation in aircraft components. The results are given in Table 1.

For comparative purposes a composition was prepared as in Example 1 but which contained as anti-erosion agent 0.05 wt % of a mixture of potassium perfluoro-octyl sulfonates (i.e., FC-98® from 3M) in lieu of KPF₆. This fluid was also subjected to the electro-deposits test and the results are given in Table 1. TABLE 1 Properties Example 1 Comp. Example 1 Electrode appearance Clean Clean Electrode 2A 2C 2A 2C Metal wt loss, mg 0.70 18.80 1.50 5.20 Deposit wt, 1st rinse, mg 10.90 2.80 13.10 3.0 Deposits wt, 1st soak, mg 10.70 3.00 10.80 2.5 Deposit wt, 2nd soak, mg 9.90 2.40 10.40 2.50 Fluid appearance after test Purple Purple Brownish Brownish

Example 2, Comparative Example 2

The anti-erosion performance of Example 1 and Comparative Example 1 were subjected to an erosion rig test according to AS1241. The tests were run at 225° F. and 275° F. and the pressure maintained at 3000 psi. The leakage rate was measured throughout the run. The results are shown in Table 2. TABLE 2 Initial Final Leakage, Leakage Delta Valve Test Flow, Flow, cc/ Run Fluid Positions Temp. cc/min cc/min min Hours Compar- 6&8 275° F. 400 355 65 524 ative 1 Example 1 5&7 225° F. 288 200 88 739

Comparative Example 3

A fluid composition was prepared containing: 78.7 wt % tributyl phosphate; 11.8 wt % isopropylated triphenyl phosphate; 6.2 wt % of a poly alkylacrylate VII; 2.3 wt % of a cyclo aliphatic diepoxide; 0.01 wt % of NH₄ PF₆ and the balance antioxidants, rust inhibitors, dye and defoamers. The composition was then subjected to the anti erosion rig test of Example 2 with the results set forth in Table 3. Included in Table 3 are the results of Example 2 using the fluid of Example 1. TABLE 3 Initial Final Leakage Leakage Test Flow, Flow, Delta Run Fluid Temp. cc/min cc/min cc/min Hours Comparative 3 225° F. 222 429 207 244 Example 1 225° F. 288 200 88 739

The data shows that the combination of KPF₆ and a mono epoxy cyclohexane carboxylate provide surprisingly lower erosion as determined by the leakage flow. 

1. A fluid composition comprising: a major amount of a phosphate ester base stock and a minor amount of an alkali metal perfluorophosphate or mixture of alkali metal perfluorophosphates and a mono epoxycyclohexane carboxylate.
 2. The composition of claim 1 wherein the alkali metal perfluorophosphate or mixtures of alkali metal perfluorophosphates are present in an amount of from about 0.01 wt % to about 0.5 wt % based on the weight of the composition.
 3. The composition of claim 2 wherein the mono epoxycyclohexane carboxylate is represented by the formula

where R is an alkyl group of 1 to 10 carbon atoms or —(CH₂)_(n)—OR, where n is an integer of 1 to 3 and R₁ is an alkyl group of 1 to 10 and wherein said mono epoxycyclohexane carboxylate is present from about 1 wt % to about 10 wt % based on the weight of the composition.
 4. A hydraulic fluid composition comprising: (a) a major amount of a phosphate ester base stock comprising, based on the weight of base stock; (i) from 10 to 100 wt % of trialkyl phosphate; (ii) from 0 to 75 wt % of dialkyl aryl phosphates; (iii) from 0 to 30 wt % of alkyldiaryl phosphates; and (iv) from 0 to 20 wt % of alkylated triaryl phosphates, and wherein the alkyl groups of (i) to (iv) have from 3 to 8 carbon atoms; and (b) both (i) from about 0.01 wt % to about 0.5 wt %, based on the weight of the composition of an alkali metal perfluorophosphate or mixtures of alkali-metal perfluorophosphate; and (ii) from about 1 wt % to about 10 wt %, based on the weight of the composition, of a compound having the formula

where R is an alkyl group of 1 to 10 carbon atoms or —(CH₂)_(n)OR, where n is an integer of 1 to 3 and R is an alkyl group of 1 to 10 carbon atoms.
 5. The composition of claim 4 wherein the alkali metal of the perfluorophosphate is potassium and R is a branched alkyl group of 4 to 8 carbon atoms.
 6. The composition of claim 5 including a poly(alkylacrylate) or poly(alkylmethacrylate) viscosity index improver (VII).
 7. The composition of claim 6 wherein the VII is present at from about 3 wt % to about 10 wt % based on the weight of the composition. 